Hydrogen energy has merits in that it is renewable and does not cause environmental pollution, and studies on hydrogen energy are actively underway.
However, there is a problem in that it is difficult for hydrogen gas to be widely applied in daily life unless the safety of use thereof is guaranteed, since hydrogen gas is vulnerable to the danger of explosion if it leaks into the surrounding atmosphere at a concentration of 4% or more. Therefore, along with studies on the utilization of hydrogen energy, the development of a hydrogen gas detection sensor (hereinafter, simply referred to as a “hydrogen sensor”), which can detect the early stages of leakage of hydrogen gas when applied in practice, is actively underway.
The hydrogen sensors that have been developed to date include ceramic/semiconductor sensors (e.g., contact combustion, thermoelectric, and semiconductor thick film sensors), semiconductor device sensors (e.g., Metal-Insulator-Semiconductor Field-Effect Transistor (MISFET) and Metal-Oxide Semiconductor (MOS) sensors), optical sensors, electrochemical sensors (e.g., potentiometric/amperometric sensors), etc.
In the case of the ceramic/semiconductor sensors, many of the ceramic/semiconductor sensors take advantage of the change of electrical conductivity that occurs when gas contacts the surface of the ceramic semiconductor. Since most of the sensors are generally used by being heated in the air, metal oxides (e.g., ceramics, SnO2, ZnO, and Fe2O3), which are stable at high temperatures, are mainly used. However, as a disadvantage, these sensors cannot detect high concentrations of hydrogen gas, since they become saturated in high concentrations of hydrogen gas.
Among these examples, the contact combustion sensor is a type of sensor that detects a change in the combustion heat generated from an oxidation reaction, which occurs through the contact of combustible gas with the surface of the sensor. This sensor has advantages in that its output is proportional to the concentration of gas, the precision of detection is high, and it is rarely influenced by ambient temperatures or humidities. However, this sensor has disadvantages in that its operating temperature has to be high and it is not selective.
In addition, in the semiconductor device sensors (e.g., MISFET and MOS sensors) and the optical sensors, which use a gasochromic material, the light transmissivity of which varies depending on the adsorption of hydrogen, palladium (Pd), which exhibits good ability to absorb hydrogen gas, is used. However, these sensors have a disadvantage in that their performance degrades when they are repeatedly exposed to high concentrations of hydrogen gas.
Finally, the electrochemical sensors are devices that electrochemically oxidize or reduce the gas that is to be detected, and measure current flowing through an external circuit at that time. These sensors can be divided into an electrostatic type, a galvanic cell type, an ion cell type, and the like depending on the principle of detection. Although these sensors have various gas detection abilities, they suffer from the disadvantage whereby the method of manufacturing the sensors is complicated and difficult.
Materials for sensors that have been recently used as hydrogen detection technologies include a Pd thin film sensor, a semiconductor sensor, which uses an MISFET or the like, a carbon nanotube sensor, a titania nanotube sensor, and the like (F. Dimeo et al., 2003 Annual Merit Review). However, despite the respective advantages of these technologies, the performance of these sensors is still dissatisfactory in terms of initial concentration of hydrogen that can be detected, response time, detection temperature, drive voltage, and the like, which can be regarded as key factors of a hydrogen sensor.
One technology that was recently developed discloses the use of the phenomenon whereby, when sites on which Pd particles can be generated are prepared using a graphite layer through the reaction between Pd and hydrogen, the resultant Pd particles are formed into a wire, in which Pd lattices are expanded and connected to each other as hydrogen is introduced into a functionalized substrate, thereby reducing electrical resistance (Penner et al. Science 293 (2001), 2227-2231). Here, the lattice expansion of Pd in response to hydrogen adsorption was experimentally observed, and an electrical signal was thereby detected by arraying the Pd nanoparticles in the form of a discontinuous wire. However, the disadvantages are that the method of manufacturing the sensor is complicated and the minimum concentration that can be detected is high.
In general, the hydrogen gas detection sensors using the Pd thin film are widely used, since they exhibit hydrogen detection performance that is far better than those of sensors manufactured using other materials. In such hydrogen sensors of the related art, a method of expanding lattices by bringing the Pd particles into close contact with the substrate by applying a strong force to the Pd particles via sputtering, vapor deposition, or the like was used. However, the sensitivity to hydrogen was not high, since the amount of expansion was reduced by the force of bonding to the substrate. In addition, in the case in which the Pd particles are not bonded to the substrate, if the exposure to hydrogen is stopped after the Pd lattices are expanded upon exposure to hydrogen, the initial state was not restored due to the bonding force between the Pd particles, thereby entailing the disadvantage of low reproducibility. Furthermore, the hydrogen sensors using Pd particles have other problems in that they react only to a high concentration of hydrogen and their initial resistance value changes when they are no longer exposed to hydrogen.
Although the hydrogen sensors of the related art have overcome some of the problems with the existing hydrogen sensors as described above, they fail as replacements for the existing sensors for reasons pertaining to detection ability, sensitivity, stability, high speed of response at low concentrations, and the like.
Therefore, studies on materials and structures that can optimize hydrogen detection performance are underway, and attempts to improve hydrogen detection performance using nanomaterials are continually being made.
Pd, as a representative nanomaterial, has a property such that it reacts with hydrogen regardless of the surrounding environment, and exhibits a phenomenon in which its lattice constant increases when it chemically absorbs hydrogen gas, thereby showing increased resistance when current is induced.
Based on this phenomenon, recent studies on solid hydrogen sensors that react only to hydrogen using Pd nanowires (NWs), the surface area of which is maximized, are actively underway. The hydrogen sensor using the Pd NWs is adapted to detect hydrogen based on the phenomenon by which the resistance of the Pd NWs changes depending on whether or not hydrogen is present.
Methods of manufacturing the Pd NWs that have been developed to date include a method using a Highly Oriented Pyrolytic Graphite (HOPG) template, a method using E-Beam lithography (EBL), a method using Dielectrophoresis (DEP), and the like.
The method of using an HOPG template is a method of electrochemically producing Pd NWs in a nanotemplate of a substrate. However, this method has disadvantages in that the manufacturing process is complicated and time-consuming and the production yield is low. The low production yield is attributable to the resultant Pd NWs, which are difficult to impart with a constant resistance value due to errors during the manufacturing process.
In addition, the method using EBL is a method of electrochemically forming Pd NWs after forming a nanopattern on a substrate. However, this method has the disadvantages of low production yield and high manufacturing cost.
The method using DEP is a method of producing NWs by forming a layer of a NW material on a substrate, followed by supplying a high-frequency AC power source through metal electrodes. However, this method also has disadvantages in that the manufacturing process is complicated and the production yield is low, since it is impossible to produce Pd NWs having a uniform shape.
Therefore, the development of a novel manufacturing process, which can manufacture a Pd hydrogen sensor at a low cost and in a simple process while ensuring that the hydrogen detection performance of Pd is maintained unchanged, is required.